Present household electric appliances mostly utilize electric motors. Take For example a light-weight duty air-conditioning system, which utilizes motors for an indoor side heat exchanger as well as an out -door side heat exchanger. These motors generally have output powers in the range of few Watts to tens of Watts, and most of them are brushless DC motors for their high power efficiency.
An example of such DC motor is an outer rotor type DC motor, as disclosed in Japanese Utility Publications 63-16297, which comprises a stator made up of a multiplicity of coils wound on respective teeth and a rotor having magnets surrounding the stator. Another type of such DC motor is an inner rotor type DC motor, which has a rotor installed inside a stator, as disclosed in Japanese Utility Publication 63-34449.
In order to manufacture compact air-conditioners, it is very important to provide them with fan motors small in size, wherein the size of a motor is given by the following formula: T=2.times.L.times.D, where T is the output torque of the motor; L is the total thickness of the multiple teeth; and D is the inner diameter of an air gap between the rotor and the stator of the motor.
Consequently, in order to obtain high-output power under a given output torque requirement, the inner air gap D should be as large as possible. Thus, outer rotor type motors generally having larger air gads D than inner rotor type motors, are superior to the latter type. This in turn implies that for a give output an outer rotor type motor may have less thickens than corresponding inner type motor. A further advantage of outer rotor type motors is that in winding coils it is much easier to work with outer rotors than with inner type rotors, requiring much fewer assembling processes.
A typical outer rotor type brushless motors has a stator molded in a thermally cured resin to reduce vibrations and noises that can arise from lose contacts of elements involved, as discussed in Japanese Patent Early Publication 4-236155.
However, such outer rotor type brushless DC motors are usually difficult to dismantle once it is molded in resin and thermally cured. Thus, defective motors such as those having manufacturing flaws or deteriorated by aging have been disposed in ground, without removing their stators. This can cause, however, serious environmental hazards. Moreover, their manufacture requires a large number of molding installations to meet large and/or spontaneous demand for such motors, since molding takes time.